The present invention relates to a process of recovering a platinum group metal from a waste catalyst having, for example, a honeycomb shaped or pelletized support formed by cordierite, alumina or the like containing the platinum group metal for purifying a waste gas of an automobile (hereinafter referred to as "SAC").
Heretofore, the following methods have been employed for recovering a platinum group metal from SAC. One method consists of extracting the platinum group metal with a solution prepared by adding an oxidant to an acid such as aqua regia, repeating the procedures of solid-liquid separation from SAC and of washing, reducing the platinum group metal by adding such a reductant as a base metal, hydrazine and sodium borohydride (SBH) to the solution containing the platinum group metal and filtering the platinum group metal. Another method consists of controlling the acidity of the solution having extracted the platinum group metal, extracting the platinum group metal with an organic solvent, back-extracting with such an appropriate solvent as ammonia and reducing with a reductant for recovery.
The extraction method employing such an acid as aqua regia possesses the following drawbacks. The extraction yield thereof is low so that the repetitive extraction procedures are required to overcome the low yield. Therefore, a large volume of an acid for extraction is wasted and the extracted platinum group metal is diluted due to the large amount of the solution employed for acid extraction. A plenty of labor and time are necessary to attain a high recovery rate by mean of reduction, that lacks profitability.
A further method is employed which consists of adding SAC as a slag component to a smelting process of copper, lead or iron, forcing the copper, the lead, the iron or their sulfides to absorb the platinum group metal by means of melting, and taking out the platinum group metal as anode slime through electrolytic refining. A still further method consists of dissolving the iron with an acid and extracting and separating the platinum group metal with an organic solvent.
The method of electrolytic refining of the copper or lead possesses the drawbacks that because not all of the copper or lead containing the platinum group metal employed as an electrode can be electrolyzed so that the remaining portion of the electrode is added to another electrode in the next process and is electrolyzed, a long period of time is required for recovering all the platinum group metal, that lacks profitability. Another drawback of the electrolyzing method resides in that because palladium and rhodium out of the platinum group metals dissolve into an electrolyte which cannot be recovered as anode slime, the palladium and the rhodium dissolved in the electrolyte must be recovered through another method.
Since, in the above process of iron smelting, it is difficult to allow the platinum group metal to exist in the iron at a high concentration, a large volume of iron into which the platinum group metal is absorbed is required so that a volume of an acid which is employed for dissolving the large amount of iron is inevitably large, that also lacks profitability.